There is an increased interest in nonvolatile memory that operates in conjunction with the volatile main system memory. Volatile memory has an indeterminate state when power is interrupted to it, and nonvolatile memory maintains its state even when power is interrupted. Nonvolatile memory is typically slower to access than volatile memory, with traditional nonvolatile memory having access speeds that are at least an order of magnitude slower than the volatile system memory. Recent developments in resistive-based memory devices provide memory that is nonvolatile and has access speeds that are comparable to the access speeds of volatile memory, instead of orders of magnitude slower.
Resistive-based nonvolatile memory can degrade over time. As the cells are cycled, the behavior of the device can change. In particular, the electrical setting characteristics will change over the life of the memory device, such as the access voltage levels or currents. There are margins between a particular electrical setting and the value needed to cause the access desired. Those margins can shift over time, which will cause the presets or trim values to not be effective.
Descriptions of certain details and implementations follow, including non-limiting descriptions of the figures, which may depict some or all examples, and well as other potential implementations.